The invention relates in the first place to a process for preparing a cylindrical rotary silk-screen printing stencil for printing, comprising the steps of applying a patterned sealing coating to the stencil and fixing end rings in the stencil in order to be able to incorporate it in a printing machine.
Such a process is generally used in the industry involved in printing materials using rotary silk-screen printing.
A rotary silk-screen printing stencil is a cylindrical metal sleeve, often made of nickel, which is provided with a large number of regularly arranged perforations. The part of such a stencil corresponding to those areas of the substrate which have to remain unprinted is sealed, while in the remaining surface of the stencil the perforations remain un-sealed. A stencil which is not provided with a patterned seal is also often called a screen, while a stencil is then the term used for a screen provided with a patterned seal. In the present invention no distinction will be made and, depending on the situation, a stencil can represent either an unsealed screen or a screen sealed in a pattern. Such a stencil with a seal in pattern form is clamped using end rings in a rotary silk-screen printing machine and subsequently, using a suitable squeegee inside the stencil, an impression of the open part of the stencil can be applied continuously to a substrate which moves in contact with the stencil at a uniform speed which is equal to the peripheral speed of the stencil in question.
In general, the stencil is first provided with the above-mentioned patterned seal, and only then are the end rings placed in the stencil.
Nowadays there is an increasing need for an accelerated process in which the patterned sealing coating on a stencil is replaced by a different patterned sealing coating.
This need for pattern changing speed has to do with the short print lengths desired for certain types of printed matter. For example, in label printing only a few thousand meters of a particular type of label will often be required; the time taken for such a print length is short, and the facility for a rapid change of pattern using a limited stock of stencils is therefore very desirable.
In such a case a stencil hitherto first had its end rings removed; the stencil was then stripped and a new patterned sealing coating was applied to the stencil again.
Such a process is very time-consuming, particularly in view of the fact that for good fixing of end rings in a stencil adhesives requiring a relatively long hardening time are used.
The object of the present invention is to produce a solution to the above-mentioned disadvantage, and to this end the invention is characterized in that in the process of the type mentioned, starting from an uncoated stencil, the end rings are first fixed and the patterned sealing coating is then formed by coating the stencil with a layer of a photographic lacquer, drying and patterned exposure thereof, and developing and hardening of the patterned sealing coating.
A very considerable saving of time is achieved by providing the stencil with end rings once, at the beginning of its service life, and leaving these end rings in the stencil during all subsequent operations, which makes a quick pattern change possible.
Use of the process according to the invention also means that the stability of the stencils during their use is improved very considerably. For the end rings give the relatively delicate stencil improved resistance to deformation, which means that damage is much less likely to occur. Since the rings stay in the stencil all the time once fitted, the total service life of the stencils is also very greatly improved.
In particular, the process according to the invention is carried out such that end rings whose greatest external diameter is smaller than the external diameter of the stencil in question are used, and for the application of the coat of photographic lacquer the stencil is accommodated between two supporting rings, which can each accommodate an end ring at one end on the inside, and in which at least at said end the external diameter is essentially equal to the external diameter of the stencil in question, and during the coating one or more coats of a liquid photographic lacquer are applied to the outside of the stencil, possibly with intermediate drying, and using a sealing collar lying against the stencil. Preferably a supporting ring which is identical at both ends and has the same external diameter over its entire length is used.
Using in particular end rings which, during the operation in which a coat of liquid photographic lacquer is being applied to the outside of the stencil, fall within the supporting rings used in that coating operation means that the so-called designing process can be carried out in all cases with a stencil already containing the end rings.
The invention also relates to a system of devices for making ready for printing a rotary silk-screen printing stencil, with which a stencil is provided with a patterned, hardened coating and end rings are fitted in order to permit placing of the stencil in a printing machine.
Such a system of devices will in general be found in a department for designing rotary silk-screen printing stencils.
According to the invention such a system is made up of one or more of the following devices: a coating device for applying a photographic lacquer coating to the stencil, an exposure device for patterned exposure of the photographic lacquer coating, a developing device for developing the coating exposed in pattern form, a hardening device for thermal hardening of the patterned coating present after developing; if necessary, a stripping device for stripping a patterned coating which may or may not be thermally hardened, and an adhesive application device for providing the stencil with end rings; the devices indicated and forming part of the system are designed in such a way that in a first adhesive application device the stencil can be provided with end rings, and each of the remaining devices of the system is designed in such a way that the stencil can be accommodated and processed together with the end rings which have been fitted.
If in such a system of devices in particular the device with which a photographic lacquer coat is applied to the outside of a stencil is designed in such a way that it can function in the presence of end rings in the stencil in question, this makes it possible to achieve a very rapid change of a patterned sealing coating on a stencil in such a system.
In such a system it is not important now whether it is a brand new stencil or a used stencil, or a stencil which has been previously subjected to a stripping operation to remove a patterned sealing coating applied earlier.
The specific aspects of the devices forming part of the system are indicated in the sub-claims and will be explained in greater detail in the discussion of the drawing below.